In recent years, there has been considerable development in biomaterials, mainly for ecological reasons.
In general, these materials are either materials reconstituted from products isolated from plants, or products isolated from plants mixed with synthetic materials, such as polymers. A commercially available product composed of a combination of a synthetic biopolymer and wheat, corn or potato starch, is an example of a material in this latter category. Several polymers may be combined with starch, such as polyvinyl chloride, polyethylene or polyvinyl alcohol.
In the first category are materials obtained by combination of starch with plant fibers.
However, these products have limited mechanical properties and are often sensitive to water.
U.S. Pat. No. 5,683,772 discloses compositions, which may be used as packaging materials, containing a starch-based binder, an inorganic filler, and fibers dispersed uniformly in the starch matrix. These fibers may include cellulose fibers, and may be obtained from leaves, stems, or other parts of the plant.
However, these fibers, and the starch, must necessarily be isolated, which considerably increases the manufacturing costs.
Application WO 95/04 111 discloses articles composed of a material containing wood particles impregnated with acid resin and a binder, which may be starch and/or proteins. In this material, the wood particles must necessarily be impregnated with resins and plant oils or fats.
U.S. Pat. No. 5,160,368 relates to a process for manufacturing packaging materials, comprising the production of a paste by heating a flour, from a graminaceous plant. This paste is mixed with crushed hay.
This process necessarily requires the production of the flour from the graminaceous plant, then its cooking, before admixture with the hay. There is thus no direct mixing of the grain and the hay.
It thus emerges from the state of the art that there is no known process for producing coherent materials from raw plant matter, in other words from plant matter not having been subjected to separation, or isolation of its different components.
One of the problems posed to a person skilled in the art lies in the heterogeneity of the components within a single plant tissue, and even more between two different tissues. In fact, in a whole cereal plant, the fibers have very different characters according to the part of the plant, both in their composition and shape. Thus, fibers of wheat straw or of the outer part of the stems and leaves of corn are long fibers, rich in cellulose (40-45% of dry matter), relatively ligneous (15% of dry matter), the remainder being composed of hemicelluloses, of xylan type structure. The fibers of corn bran or wheat bran (grain coating) are very different, being short, elastic, much richer in hemicelluloses (up to 60% of dry matter of the fibers against 15% of cellulose and 8% of lignin). In addition, these hemicelluloses are of arabinoxylan type, much more highly substituted than the stem xylans, and with thickening and gelling properties in solution in water, in fact filmogenic properties. The fibers of the outer part of the corn cob (hard part) are very different from those of the central or inner part (tender or soft part). The former are very hard, proportionally richer in cellulose (47%) and in lignin (7%) and less in hemicelluloses (37%), while the latter are soft, proportionally less rich in cellulose (35%) and in lignin (5%). The water-absorbent power of the soft fibers is seven times greater than that of the hard fibers.